Recently the Moroccan government expelled a group of Christian missionaries in what some have seen as the beginning of surreptitious moves to remove all missionaries. It has hardly caused a stir. And considering the outrage which accompanied the missionaries who attempted to "adopt" Haitian children shortly after the earthquake there you might be tempted to assume that missionaries are a parasitical leftover of the imperial age.

And I can understand that. There is a lot of evidence that the huge missionary movements from Europe in the 18th and 19th centuries displaced indigenous cultures and were little more than extensions of the military colonisation which left the inhabitants impotent when they finally withdrew. And we are just as wary of the impositions of current day missionaries. I still recall visiting a missionary colleague in Nigeria some years ago and the shock I experienced when Nigerian pastors turned up for the official photograph looking more British than the British! The Hindu Council of Britain is still apoplectic about missionaries who try to "convert" the dalits of India, challenging and disrupting the caste system. Philosophically, these Hindus and the secularists have one thing in common: conversion – pejoratively known as "proselytising" – is a very bad idea. And that apparently, is what missionaries do. Or is it?

True enough, missionaries of all types and faiths have a real sense of purpose which is loaded with ideas of transcendence and the notion that they can offer other people a better way to live their lives.

But the 18th and 19th centuries were not all bad for those who received missionaries. Missionaries were invariable people propelled by a passion for people – not just their mission. Christian mission is quintessentially fuelled by the idea that God loves everyone, offers life through Jesus Christ and that that life carries with it the gift of spiritual and social wellbeing. And usually missionaries believe this so much that for 2000 years they have been prepared to leave comfort and family, risking disease and death to offer others a better way to live. Sixteen hundred years ago Christian bishops would send the parabolani into plague-ridden communities without any expectations that they might return from their mission.

Missionaries have established healthcare, orphanages, education and built infrastructure. Like Albert Schweitzer and William Carey they have been educators, philosophers, scientists, anthropologists and humanitarian workers. Like John Wesley, Count Zinzendorf, Olaudah Equiano and John Smith of Demerara, they were at the forefront of the abolitionist movement. Despite the outrageous episodes of child abuse and the fanciful, demonising mythologies which Dan Brown has spawned in our public consciousness, the Catholic church has the longest unbroken track record for spreading the common good. Long before Coca Cola or the "McDonaldisation" of the world, missions were been a trans-cultural global movement for good – imperfect but also impactful.

The idea that missionaries prey on vulnerable and poor nations is outdated and misguided for two key reasons. First, recent studies in the US have shown that only 14% of work done by missionaries from some of the largest and most evangelical churches is about conversion and as much as 96% of those who go on short-term mission trips visit other nations such as Guatemala where most of the people are already Christians. And secondly, the missionary world has been tipped on its head: most missionaries now come from the global south where Africans, Latin Americans and Asians – notably from South Korea – provide an endless stream of missionaries to Europe and the Americas who are committed to return the favour. The largest and most influential independent church in the Ukraine is led by a Nigerian. The empire strikes back!

The expulsion of missionaries from a poor nation is not just an act of hostility against a foreign power; it is in effect an act of social vandalism against your own people and has always been an unsustainable and politically retrograde step. It has failed in China, in Albania and Mongolia. And this is precisely why the World Health Organisation readily acknowledges the indispensible role of faith in healthcare. In the aftermath of Hurricane Katrina in 2005 Roy Hattersley was effusive about the work of the Salvation Army. And in 2008 it was the fruit of the missionary enterprise which led the self-confessed atheist and columnist, Matthew Parris to famously declare that "Africa needs God".